Composition of the understory in 132 woody weed populations and implications for succession

Abstract

The species composition of the understory can be a key indicator of successional trajectories in the absence of disturbance at forested sites. We surveyed species composition and percent cover in the understory of 132 closed-canopy stands of 41 woody weed species throughout New Zealand as a first step in understanding potential successional trajectories in these weed populations. Twenty-seven weed species had zero, or very few, conspecific seedlings or saplings present beneath their own canopy. Fourteen weed species had medium to high numbers of conspecific seedlings and/or saplings present. Some weed species had variable understory regeneration, with high numbers of conspecific seedlings and saplings present at some sites, but none at others. Twenty-eight weed species had native understory cover of ≥ 50% at one or more sites. Native understory cover was higher at sites close to remnants of native vegetation compared to sites distant from native vegetation. Overall, many more native than non-native species were present in woody weed understories. Melicytus ramiflorus (māhoe) was the most common native species, present at 67% of sites. At least 76 other native species were recorded at five or more sites. Our results demonstrate that (1) woody weed species vary in the extent to which they regenerate under their own canopy, and (2) closed canopy woody weed stands frequently have a predominantly native understory. Further research to determine whether the composition of the understory can be used to predict successional trajectories in woody weed populations would be valuable.

Understory vegetation provides clues to succession in woody weed stands

“…However, there may be reasons other than shade-intolerance that explain why a species does not regenerate under its own canopy. In New Zealand for example, the shade-tolerant woody weed species sycamore (Acer pseudoplatanus) and tree privet (Ligustrum lucidum) regenerate under their own canopy at some sites, but not at others (McAlpine et al 2018). A potential explanation for this variability in regeneration across sites is that interspecific competition varies across one or more resource gradients (D'Antonio & Chambers 2006).…”

“…However, as we are not concerned with predicting the persistence of individual native species, there only needs to be a mix of seedlings and saplings of native species as a group (rather than seedlings and saplings of each species). In a survey of 132 woody weed stands around New Zealand, McAlpine et al (2018) recorded as many as 24 native species in the understory at some sites, and 55 sites had ten or more native species in the understory (KGM unpubl. data).…”

“…In woody weed stands, this means that the composition of the understory (i.e. the identity and abundance of resident plants) can indicate whether the woody weed is likely to persist in the long-term or be replaced naturally as succession proceeds (McAlpine et al 2018).…”

Values of weedy regrowth for rainforest restoration

“…Although there were few mature rainforest trees growing among the privet regrowth, a larger number (although still modest, and limited by cattle grazing) were present as seedlings, especially of bird‐dispersed species. Conversely, McAlpine et al () reviewed several cases of low native seedling recruitment in privet stands and concluded that privet was not a useful regeneration facilitator.…”

“…Management decisions could then draw on an evidence‐based understanding of their ecological roles, and better information about whether these roles are environmentally harmful or useful (see, e.g. McAlpine et al ()).…”